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DOE OSTI2022

THERMAL MODELING OF HANFORD LEAD CANISTER’S HEATER BENCH TESTS

A computational fluid dynamics (CFD) model was built to simulate planned testing of heater assemblies for the Hanford Lead Canister (HLC) project. The HLC is a canister storage system that will contain heaters to simulate the decay heat of nuclear material and provide the canister storage system with environmental conditions equivalent to the operating conditions on a dry storage pad. The HLC will be equipped with long-term data collection and monitoring systems to provide an early warning of corrosion, pitting, cracking, or other signs of canister degradation that might threaten the integrity of the containment boundary over the potentially long term of dry storage. An important part of the HLC development is to confirm the function and ability of the electric heater assemblies that were specially designed to provide heating similar to the decay heat of nuclear material contained within the canister storage system. Heater bench testing is planned for early 2022 in a test configuration that does not include the canister. The goal of the bench testing is to verify that the heaters can replicate the decay heat of a canister with nuclear material and to validate the thermal models, which are critical to understanding the HLC’s thermal environment, including the local air flow within the canister storage system. Testing of the heater assemblies inside the canister system are planned in the future to validate canister level thermal models, and rigorous pre-deployment testing of the complete HLC cask and canister system is intended to be completed before the HLC is deployed in the 2025-2026 timeframe. This study presents the pre-test temperature predictions of the bench testing. A description of the heater assemblies and planned bench testing is presented. The model was developed with the commercial CFD code STAR-CCM+. An uncertainty analysis was run with the CFD model to determine the uncertainty in the temperature predictions and provide a range over which the predicted temperatures are expected to vary. The uncertainty analysis was performed by coupling STAR-CCM+ with the software Dakota, which provides advanced parametric analyses, including quantification of margins and uncertainty with computational models. This work is expected to provide insight into SNF canister behavior.

Suffield, Sarah R.↗

DOE OSTI2024

THERMAL MODELING OF HANFORD CESIUM AND STRONTIUM CANISTERS DURING SIMULATED LOADING

A computational fluid dynamics (CFD) model was built to simulate planned testing of heater assemblies within a canister and overpack for the Hanford Lead Canister (HLC) project. The HLC is a canister storage system that will contain heaters to simulate the decay heat of nuclear material and provide the canister storage system with environmental conditions equivalent to the operating conditions on a dry storage pad. The HLC will be equipped with long-term data collection and monitoring systems to provide an early warning of corrosion, pitting, cracking, or other signs of canister degradation that might threaten the integrity of the containment boundary over the potentially long term of dry storage. An important part of the HLC development is to make pretest numerical predictions for the behavior of the heated canister during the simulated radiolytic decay heat testing, which simulates the dry storage system during loading operations. The simulated radiolytic decay heat test is planned for mid-2024 in a configuration that includes the heater assembly, overpack, and canister, but with the lids removed to allow loading cesium and strontium capsules into the canister. One of the goals of the test is to evaluate the thermal behavior of the canister and overpack assembly in the ambient air of the test facility, which will provide data critical to validating the thermal models and understanding how the HLC will perform as a system once deployed. To best approximate real-world conditions, the CFD model includes the full air volume of the mock-up truck bay the heated canister test will be performed in, enabling detailed investigation of how the heated canister affects airflow around it. Rigorous pre-deployment testing of the complete HLC cask and canister system is intended to be completed before the HLC is deployed in the 2028 timeframe. This study presents the pre-test temperature predictions of the simulated radiolytic decay heat test. A description of the heater assembly, canister, and overpack system is presented. The model was developed with the commercial CFD software STAR-CCM+. An uncertainty analysis was run with the CFD model to determine the uncertainty in the temperature predictions and provide a range over which the predicted temperatures are expected to vary. The uncertainty analysis was preformed by coupling STAR-CCM+ with the software Dakota, which provides advanced parametric analyses, including quantification of margins and uncertainty with computational models. This work is expected to provide insight into SNF canister behavior.

Carpenter-Graffy, Dina E.↗

DOE OSTI2015

TWO-DIMENSIONAL CORE-COLLAPSE SUPERNOVA MODELS WITH MULTI-DIMENSIONAL TRANSPORT

We present new two-dimensional (2D) axisymmetric neutrino radiation/hydrodynamic models of core-collapse supernova (CCSN) cores. We use the CASTRO code, which incorporates truly multi-dimensional, multi-group, flux-limited diffusion (MGFLD) neutrino transport, including all relevant O(v/c) terms. Our main motivation for carrying out this study is to compare with recent 2D models produced by other groups who have obtained explosions for some progenitor stars and with recent 2D VULCAN results that did not incorporate O(v/c) terms. We follow the evolution of 12, 15, 20, and 25 solar-mass progenitors to approximately 600 ms after bounce and do not obtain an explosion in any of these models. Though the reason for the qualitative disagreement among the groups engaged in CCSN modeling remains unclear, we speculate that the simplifying ''ray-by-ray'' approach employed by all other groups may be compromising their results. We show that ''ray-by-ray'' calculations greatly exaggerate the angular and temporal variations of the neutrino fluxes, which we argue are better captured by our multi-dimensional MGFLD approach. On the other hand, our 2D models also make approximations, making it difficult to draw definitive conclusions concerning the root of the differences between groups. We discuss some of the diagnostics often employed in the analyses of CCSN simulations and highlight the intimate relationship between the various explosion conditions that have been proposed. Finally, we explore the ingredients that may be missing in current calculations that may be important in reproducing the properties of the average CCSNe, should the delayed neutrino-heating mechanism be the correct mechanism of explosion.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2025

Gas and dust dynamics in γ Cephei-type disks

Context. Giant planets are observed orbiting the primary stars of close binary systems. Such planets may have formed in compact circumprimary disks, which once surrounded these stars, under conditions much different than those encountered around single stars. Aims. In order to quantify the effects of the strong gravitational perturbations exerted on circumprimary disk material, the three-dimensional (3D) dynamics of gas and dust in orbit around the primary star of a compact and eccentric binary system was modeled by applying the stellar and orbital parameters of γ Cephei, a well-known system that can be representative of a class of close binaries. Methods. Circumprimary gas was approximated as an Eulerian viscous and compressible fluid and modeled by means of 3D hydrodynamical simulations, assuming locally isothermal conditions in the medium around the primary star. Dust grains were modeled as Lagrangean particles, subjected to gravity and aerodynamic drag forces. Models that include a giant planet were also considered. Results. Models indicate that spiral density waves excited around pericenter passage propagate toward the inner boundary of the disk, through at least a few pressure scale-heights from the mid-plane, inducing radial and vertical mixing in the gas. However, perturbations imparted to gas, both in terms of eccentricity and precession, are far weaker than previously estimated by two-dimensional (2D) simulations. Models predict small eccentricities, ≲0.03, and slow retrograde precession. The addition of a giant planet does not change the low eccentricity state of the disk. The parameters applied to the disk would lead to the formation of a massive planet, many times the mass of Jupiter, in agreement with some observations. Micron to mm-size dust grains are well coupled to the gas, resulting in similar dynamics and statistically similar distributions of orbital elements. The planet only affects the dust distributions locally. In agreement with outcomes of recent 2D models, the lifetime of an isolated circumprimary disk would be brief, ~10 5 years, because of its compact nature, requiring a long-term external supply of mass to allow for the in situ formation of a giant planet.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

A uniform history for galaxy evolution

Recent observations indicate a remarkable similarity in the properties of evolving galaxies at fixed mass and redshift, prompting us to consider the possibility that most galaxies may evolve with a common history encompassing star formation, quasar accretion, and eventual quiescence. We quantify this by defining a synchronization timescale for galaxies as a function of mass and redshift that characterizes the extent to which different galaxies of a common mass are evolving in the same manner at various cosmic epochs. We measure this synchronization timescale using nine different star-forming galaxy observations from the literature and Sloan Digital Sky Survey quasar observations spanning 0 < z ≲ 6. Surprisingly, this synchronization timescale is a constant, approximately 1.5 Gyr for all combinations of mass and time. We also find that the ratio between the stellar mass of galaxies turning off star formation and black hole mass of turnoff quasars is approximately 30:1, much lower than the 500:1 for quiescent galaxies at low redshift. As a result, we propose a model in which the star-forming 'main sequence', analogous quasar behavior, and other observations form a galactic evolution 'main sequence', in which star formation occurs earliest, followed by supermassive black hole accretion, and feedback between the two are dominated by deterministic rather than stochastic processes.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1985

Molecular cloud evolution and star formation

The present state of knowledge of the relationship between molecular clouds and young stars is reviewed. The determination of physical parameters from molecular line observations is summarized, and evidence for fragmentation of molecular clouds is discussed. Hierarchical fragmentation is reviewed, minimum fragment scales are derived, and the stability against fragmentation of both spherically and anisotropically collapsing clouds is discussed. Observational evidence for high-velocity flows in clouds is summarized, and the effects of winds from pre-main sequence stars on molecular gas are discussed. The triggering of cloud collapse by enhanced pressure is addressed, as is the formation of dense shells by spherical outflows and their subsequent breakup. A model for low-mass star formation is presented, and constraints on star formation from the initial mass function are examined. The properties of giant molecular clouds and massive star formation are described. The implications of magnetic fields for cloud evolution and star formation are addressed.

Silk, J.↗

NASA NTRS1986

The Oort cloud in transition

The evolution of theoretical and empirical models of the Oort cloud (OC) since it was first proposed by Oort in 1950 is traced, and the main features of current models are discussed, in a general review. Consideration is given to work on the classical OC (Monte Carlo simulations of OC evolution, population and mass estimates, and OC perturbation by passing stars and giant molecular clouds), models of a massive inner OC (simulations of planetesimal-swarm evolution in the Uranus-Neptune zone and IRAS observations of circumstellar dust shells), evidence for random and/or periodic comet showers, and the possible role of the Galactic missing mass. The current OC model comprises an almost spherical outer (10,000-100,000-AU) cloud of mass 7-8 earth mass and population (1.4-2.3) x 10 to the 12th, and a flat disklike inner (40-10,000 AU) cloud of mass 100-200 earth mass and population (1-10) x 10 to the 13th.

Weissman, P. R.↗

NASA NTRS2001

Flash Mixing on the White-Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

Brown, Thomas M.↗

DOE OSTI2021

Stellar migration and chemical enrichment in the milky way disc: a hybrid model

ABSTRACT We develop a hybrid model of galactic chemical evolution that combines a multiring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc’s abundance structure: (i) the dependence of the [O/Fe]–[Fe/H] distribution on radius Rgal and mid-plane distance |z|; (ii) the changing shapes of the [O/H] and [Fe/H] distributions with Rgal and |z|; (iii) a broad distribution of [O/Fe] at sub-solar metallicity and changes in the [O/Fe] distribution with Rgal, |z|, and [Fe/H]; (iv) a tight correlation between [O/Fe] and stellar age for [O/Fe] > 0.1; (v) a population of young and intermediate-age α-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age–[O/H] and age–[Fe/H] relations, with large scatter and a median age of ∼4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate ∼2 Gyr ago improve agreement with the observed age–[Fe/H] and age–[O/H] relations, but worsen agreement with the observed age–[O/Fe] relation. None of our models predict an [O/Fe] distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice).

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

SOLAR OPACITY CALCULATIONS USING THE SUPER-TRANSITION-ARRAY METHOD

A new opacity model has been developed based on the Super-Transition-Array (STA) method for the calculation of monochromatic opacities of plasmas in local thermodynamic equilibrium. The atomic code, named STAR (STA-Revised), is described and used to calculate spectral opacities for a solar model implementing the recent AGSS09 composition. Calculations are carried out throughout the solar radiative zone. The relative contributions of different chemical elements and atomic processes to the total Rosseland mean opacity are analyzed in detail. Monochromatic opacities and charge-state distributions are compared with the widely used Opacity Project (OP) code, for several elements near the radiation–convection interface. STAR Rosseland opacities for the solar mixture show a very good agreement with OP and the OPAL opacity code throughout the radiation zone. Finally, an explicit STA calculation was performed of the full AGSS09 photospheric mixture, including all heavy metals. It was shown that, due to their extremely low abundance, and despite being very good photon absorbers, the heavy elements do not affect the Rosseland opacity.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars

We present constraints on the nature of axions and axion–like particles (ALPs) by analyzing gamma–ray data from neutron stars using the Fermi Large Area Telescope. In addition to axions solving the strong CP problem of particle physics, axions and ALPs are also possible dark matter candidates. We investigate axions and ALPs produced by nucleon–nucleon bremsstrahlung within neutron stars. We derive a phenomenological model for the gamma–ray spectrum arising from subsequent axion decays. By analyzing 5 years of gamma-ray data (between 60 MeV and 200 MeV) for a sample of 4 nearby neutron stars, we do not find evidence for an axion or ALP signal, thus we obtain a combined 95% confidence level upper limit on the axion mass of 7.9×10 -2 eV, which corresponds to a lower limit for the Peccei-Quinn scale fa of 7.6×10 7 GeV. Our constraints are more stringent than previous results probing the same physical process, and are competitive with results probing axions and ALPs by different mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

On the Anticorrelation between Duration and Redshift in Gamma-Ray Bursts

Abstract For gamma-ray bursts (GRBs) with durations greater than two seconds (so-called long GRBs), the intrinsic prompt gamma-ray emission appears, on average, to last longer for bursts at lower redshifts. We explore the nature of this duration–redshift anticorrelation, describing systems and conditions in which this cosmological evolution could arise. In particular, we explore its dependence on the metallicity of a massive star progenitor, because we can securely count on the average stellar metallicity to increase with decreasing redshift. Although stars with higher metallicity/lower redshift lose mass and angular momentum through line-driven winds, in some cases these stars are able to form more extended accretion disks when they collapse, potentially leading to longer-duration GRBs. We also examine how this duration–redshift trend may show up in interacting binary models composed of a massive star and compact object companion, recently suggested to be the progenitors of radio-bright GRBs. Under certain conditions, mass loss and equation-of-state effects from massive stars with higher metallicity and lower redshift can decrease the binary separation. This can then lead to spin-up of the massive star and allow for a longer-duration GRB upon the massive star’s collapse. Finally, the duration–redshift trend may also be supported by a relatively larger population of small-separation binaries born in situ at low redshift.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1989

Archival research on absorption lines in violently star-forming galaxies

A computerized analysis of a starburst model is discussed. The model proposes that the absorption line equivalent width should scale with the level of star forming activity. Archival International Ultraviolet Explorer (IUE) data on IUE spectra of luminous blue galaxies were compared with previous IUE observations of extragalactic HII regions and low luminosity galaxies. The comparisons are summarized and causes for offsets are discussed.

Gallagher, J. S.↗

NASA NTRS1997

Studies of Disks Around the Sun and Other Stars

This is a NASA Origins of Solar Systems research program, and this NASA Headquarters grant has now been transferred to a new grant at NASA GSFC (NAG5-4082). Thus the need for this 'Final Report' on a project that is not, in fact, complete. We are conducting research designed to enhance our understanding of the evolution and detectability of comet clouds and disks. This area holds promise for also improving our understanding of outer solar system formation, the bombardment history of the planets, the transport of volatiles and organics from the outer solar system to the inner planets, and to the ultimate fate of comet clouds around the Sun and other stars. According to "standard" theory, both the Kuiper Belt and the Oort Cloud are (at least in part) natural products of the planetary accumulation stage of solar system formation. One expects such assemblages to be a common attribute of other solar systems. Our program consists of modeling collisions in the Kuiper Belt and the dust disks around other stars. The modeling effort focuses on moving from our simple, first-generation, Kuiper Belt collision rate model, to a time-dependent, second-generation model that incorporates physical collisions, velocity evolution, dynamical erosion, and various dust transport mechanisms. This second generation model is to be used to study the evolution of surface mass density and the object-size spectrum in the disk.

Stern, S. Alan↗

NASA NTRS1980

Theoretical profiles for the 1-0 S/1/ H2 line in carbon stars

The spectra of carbon stars have been synthesized from the models of Querci, Querci, and Tsuji and Querci and Querci in the region of the 1-0 S(1) vibration-rotation quadrupole line of H2. The line is shown to be sufficiently strong to be seen against the numerous lines of the CN red system for models with effective temperatures less than about 2700 K. The usefulness of the line as a diagnostic of the atmosphere is discussed, and a comparison with the measured spectra of UU Aur and S Cep is made. It is concluded that the outer atmospheric layers of carbon stars are significantly warmer than the models predict. An additional opacity source in the outer layers is required.

Goorvitch, D.↗

NASA NTRS1986

Theories of mass loss from T Tauri stars

The available database on T Tauri stars is examined in order to develop criteria for modeling the physical processes occurring within and around pre-main sequence stars. Observational data on the temperature structures, mass loss rates, and mass motions in T Tauri envelopes are reviewed to form a basis for discussing the possible mechanisms for mass loss in the T Tauri objects. Analytical models are explored for thermally-driven, wave-driven and rotationally-driven winds. The possible interactions of the T Tauri stellar winds with processes occurring in the nearby FU Dri stars are noted, as is the necessity of momdeling physical processes such as turbulent velocity fields through accurate radiative transfer calculations in order to model the envelopes of the T Tauri stars.

Hartmann, L.↗

NASA NTRS1975

The He II lines in the spectrum of zeta Puppis

The observed He II line spectrum in zeta Pup is compared with that predicted from model atmospheres in an attempt to establish the parameters (effective temperature, log g) which characterize the model that best represents this star's atmosphere. The study is conducted by compiling equivalent widths of the He II lines in the series (n = 2,3,4,5) and comparing them with predictions from plane-parallel static model atmospheres using a non-LTE theory of line formation. Good agreement between observation and prediction for a model atmosphere with an effective temperature of 50,000 K and log of 4.0 is found for the upper members of the n = 3 and n = 5 series, but discrepancies are found for the two lines of the n = 2 series and the upper members of the n = 4 series. It is shown that the n = 4 level appears to be more overpopulated at moderate atmospheric depths than the non-LTE calculations with plane-parallel layers indicate. It is suggested that this may be due to an overlap of the H and He II lines in the even-even series caused by macroturbulent velocities of the hydrogen and helium atoms.

Snijders, M. A. J.↗

DOE OSTI2021

The radio loudness of SDSS quasars from the LOFAR Two-metre Sky Survey: ubiquitous jet activity and constraints on star formation

ABSTRACT We examine the distribution of radio emission from ∼42 000 quasars from the Sloan Digital Sky Survey, as measured in the LOFAR Two-metre Sky Survey (LoTSS). We present a model of the radio luminosity distribution of the quasars that assumes that every quasar displays a superposition of two sources of radio emission: active galactic nuclei (jets) and star formation. Our two-component model provides an excellent match to the observed radio flux density distributions across a wide range of redshifts and quasar optical luminosities; this suggests that the jet-launching mechanism operates in all quasars but with different powering efficiency. The wide distribution of jet powers allows for a smooth transition between the ‘radio-quiet’ and ‘radio-loud’ quasar regimes, without need for any explicit bimodality. The best-fitting model parameters indicate that the star formation rate of quasar host galaxies correlates strongly with quasar luminosity and also increases with redshift at least out to z ∼ 2. For a model where star formation rate scales as $L_{\rm bol}^{\alpha } (1+z)^{\beta }$, we find α = 0.47 ± 0.01 and β = 1.61 ± 0.05, in agreement with far-infrared studies. Quasars contribute ≈0.15 per cent of the cosmic star formation rate density at z = 0.5, rising to 0.4 per cent by z ∼ 2. The typical radio jet power is seen to increase with both increasing optical luminosity and black hole mass independently, but does not vary with redshift, suggesting intrinsic properties govern the production of the radio jets. We discuss the implications of these results for the triggering of quasar activity and the launching of jets.

79 ASTRONOMY AND ASTROPHYSICS↗